1. Field of the Invention
This invention relates to compounds for use in organic light emitting diodes, such as for host materials.
2. Description of the Related Art
White organic light emitting devices (WOLEDs) have attracted much attention and been intensively studied due to their potential applications as backlight sources, full color displays, and general lighting. Among various device configurations to produce white light, a single-emissive-layer device employing phosphorescent materials in combination with proper host materials is desirable. Some advantages of such device may include reduced overall cost, increased quantum efficiency and easier fabrication. Since phosphorescent emitters can harvest both singlet and triplet excitons, it may lead to the potential of achieving 100% internal quantum efficiency. Adding host materials may also reduce concentration quenching of the emissive materials and further increase the efficiency. In addition, adding host materials reduces the required amount of expensive emissive material, and the fabrication of a single layer device is easier and more cost effective than a multiple layer device. As a result, the single-emissive-layer device with phosphorescent and host materials can lower the overall cost of fabricating the WOLEDs.
The use of effective host materials is important in making efficient WOLEDs. A host may be improved if it transports both holes and electrons efficiently at the same speed. A host may also be improved if its triplet energy is high enough to effectively confine the triplet excitons on the guest molecules. Most currently used host materials are a mixture of hole-transport material and electron-transport material, which may pose potential problems of phase separation, aggregation and lack of uniformity, and unequal material degradation rates. Thus, development of an ambipolar single molecule (i.e., a molecule capable of transporting both holes and electrons effectively) for a host material would be useful.
Some ambipolar single molecule hosts have been used in either single colour or white OLED device applications. However, these molecules have either unbalanced hole-transport and electron-transport properties, or the devices made from these molecules have only moderate efficiency.
Thus there is a need for a new type of ambipolar host that can be easily synthesized, possesses high thermal and electrochemical stability, and has well balanced hole-transport and electron-transport mobility when used as a host for phosphorescent emissive materials. Such a host may be used to achieve a simple device structure with high quantum efficiency and low turn-on voltage.